Volume control system



Nov. 22, 1932. A, w HORT N, R 1,888,227

VOLUME CONTROL SYSTEM Filed Aug. 29, 1931 2 Sheets-Sheet 1VARlO-REP/gTER GAIN INCREASE/i FIG./

emu-menus: 01545459 9 GAIN DECREA 55R i l l l INVEN TOR A. W HORTON JR.

A TTORNEV Nov. 22, 1932. A. w. HORTON, JR 1,888,227

VOLUME CONTROL SYSTEM Filed Aug. 29. 1951 2 Sheets-Sheet 2VARlO-REPE'ATER INCREASE? GAIN-INCREASE DISABLE R //v VENTOR A. W. HORTON JR.

A TTORNE Y Patented Nov. 22, 1932 UNITED" STATES PATENT oFFrcs ARTHUR W.HORTON, JR., .0 EAST ORANGE, NEW JERSEY, AS S IGNOR TO BELL TELE- PHONELABORATORIES, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEWYORK VOLUME oou'raor. SYSTEM Application filed August 29, 1931. SerialNo. 560,131.

This invention relates to signaling. systems and particularly to thecontrol ofthe volume level of signals transmitted over such systems. -1

An object of the invention is to improve the operation of circuits forautomatically controlling the volume level of signals in signaltransmission systems subject to noise.

Another and more specific object is to prevent false operation of acircuit for automatically controlling the gain in a signal.

transmission path in accordance with the volume level of the signalstransmitted thereover, by noise energy insaid path without reducing theeffectiveness of the gain control circuit as regards operation by usefulsignals. i

In signal transmission systems, itis often desirable to maintain thesignal volume level within definite limits at one or more points. Forexample, in a telephone system in which the speech volume delivered atthe input varies within wide limits, it is desirable to maintain asubstantially constant volume at some given point to insure propervolume at the receiving end of the system, or to insure proper operationof associated voice-operated echo suppressors or anti-singing devices.In certain systems of the prior art this is accomplished by utilizingthree-electrode space discharge amplifying devices as loss elements inthe signal transmission paths, and automatically controlling thepotentials on the grids of these devices by adjusting the charge oncondensers in the grid circuits in accordance with the volume level ofthe signals in said paths to adjust the gain of the space dischargedevices in proper manner. Control circuits of this type are disclosed,

. for example, in the copending applications of S. Doba, Serial No.443,640, filed April 12, 1930, and Hogg and Doba, Serial No. 445.543,filed April 19, 1930..now Patent No. 1,854,- 828, issued April 19. 1932,and Patent'No.

. 1,853,974, issued April 12, 1932, respectively.

In a special form of the above mentioned general type of circuit towhich the invention is especially applicable, a portion of the signalenergy in the input ofthe amplifier in the signal path is utilizedtocontr0l a relay which operates when the input level thereto exceeds a.given minimum value, to increase the gain of the amplifier; and aportion of.

the signalenergy in the output of the amplifier when'the signal levelthe'reat exceeds a given maximum is utilized to'decrease the gain of theamplifier." The efle'ctiveness of such a volume control system of thegeneral type described above is limited by the noise present in thesystem, for the signal controlled device must not be falsely operated bynoise when signal currents are not being transmitted over the 'system..As is Well known, the noise in a long signal transmission circuit variesconsiderably from time to time. If the sensitivity of each portion ofthe volume adjusting circuit is adjusted so that it will not be operatedby the maximum noise to be expectedon the signal transmission path,theresponse of the volume con-.

trol circuit to signals might be so sluggish that the circuit would notbe suitable for-use in high quality systems requiring a gain adjustmentfor small variations in the signal level.

In accordance with the present invention,

in a system of the above mentioned type means areprovided for preventingfalse operation of the volume control circuit by noise to increase thegain in the signal transmission path, without altering the'efiectivenessof the circuitto decrease the gain in. the signal transmission pathwhenever a signal overrides'the noise. This is accomplished in oneembodiment by utilizing the noise energy in said path in the input -ofthe amplifier, as the noise level increases, to'control a chain ofauxiliary relays similar to those disclosed in the co-pending patentapplication of Bjornson-Horton-Norwine, Serial. No. 480,940, filedSeptember 10, 1930, now Patent No. 1,880,715, issued October 4, 1.932,and to decrease the sensitivity of the gain increase relay only to thepoint where it is inoperative by the noise alone, although operative by'the superimposed signal energy.

Theinvention will be better understood from the following detaileddescription thereof when read in connection with the acshowschematically volume control circuits embodying different modificationsof the invention.

Fig. 1 of the drawings shows a multi-stage vacuum tube amplifier circuit1 connecting an incoming signal transmission circuit 2 to an outgoingsignal transmission circuit 3. The first stage of the amplifier circuit1 which will be referred to hereafter as the Vario-repeater, comprisestwo three-electrode space discharge amplifying tubes 4 and 5 (which arepreferably matched to prevent distortion) connected in push-pullrelationship. The in put circuit of the push-pull tubes 4 and 5 isconnected to the incoming transmission circuit 2 by the inputtransformer 6 and the output circuit of the push-pull tubes is connectedto the input circuit of the three-electrode space discharge amplifyingtube 7 in the second stage of the amplifier by the interstagetransformer 8. The output circuit of the tube 7 is connected to theinput circuit of the three-electrode space discharge amplifying tube 9in the last stage of the amplifier circuit by the interstage transformer10, and the output circuit of the tube 9 is connected to the outgoingsignal transmission circuit 3 by the output transformer 11.

- Space current is supplied by the battery 12 to theplates of thepush-pull tubes 4 and 5 respectively through the upper and lowerportions of the resistance potentiometer 13, and to the plates ofamplifying tubes 7' and 9 through the primary winding of interstagetransformer 10 and the primary winding of output transformer 11,respectively. The battery 14' supplies heating current to the filamentsof tubes 4, 5, .7 and 9 in'series. Biasing potential is supplied to thegrids of the push-pull tubes 4 and 5 by battery 15, and biasingpotential for the grids of tubes 7 and 9 is provided by the drop acrossresistances in the filament heating circuits for the two tubes.

The plate impedance of the push-pull tubes 4 and -5 which determines thegain of the varlo-repeater is controlled bythe voltage across thecontrol condenser 17 common to the grid circuits of the two tubes. Whenthe condenser 17 is charged bythe control voltage, the potentials on thegrids of the pushpull tubes in the vario-repeater are made morenegative, thus effectively decreasing the gain of the vario-repeater.

Connected across the outgoing transmission circuit 3 in the output ofthe amplifier circuit 1 is the input of a control circuit 16 comprisingtwo parallel branches 18 and 18 which will be referred to hereafter asthe gain decreaser and the gain-increase disabler, respectively. Thebranch 18 comprises the three-electrode vacuum tube amplifying-device 19and a highly biased fullwave, space discharge rectifier 20 connected intandem, the output'of the rectifier 20 being permanently connectedacross the control condenser 17 common to the grid circuits of thepush-pull tubes 4 and 5 in the amplifier 1 so that the condenser ischarged by the rectified currents in the output of that rectifier whenthe amplitude level of the current applied to the input of the gaindecreaser increases above a predetermined value which will cause itsoperation.

The sensitivity of the gain decreaser by suitable choice of its circuitelements is made approximately the same as the initial sensitivity ofthe gain-increase disabler to be described below. This sensitivity ismade such that whenever the average amplitude level of the amplifiedsignal currents in the output of the amplifier 1 exceeds the pre-.

determined value, that is, the level at which the output signal energyof the amplifier 1 is to be maintained substantially constant, thesignal currents diverted into control circuit 16 cause the gaindecreaser to be actuated, and thus the condenser 17 to be charged so asto apply a negative bias to the grids of the tubes 4 and 5 in thevario-repeater. This results in a continuously decreasing gain until theoutput level of the amplifier falls below the operating value of thegain decreaser.

The control branch 18 which will be referred to hereafter as thegain-increase disabler comprises the three-electrode vacuum tubeamplifying tube 23 and the three-electrode space discharge detector tube24 connected in tandem. Connected between the plate and filamentelectrodes of the rectifier tube 24 is the operating winding 25 of adisabler relay 26. The disabler relay 26 has, in addition to itsoperating winding 25, a biasing winding 27, which is normally maintainedenergized by currents from the battery 28 through the resistance 29 soas to hold the relay contacts 3() closed to connect the dischargeresistance R to ground. The detector tube 24 is preferably a gas-filledtrigger tube, so that the gain-increase disabler is a marginal triggerdevice. The disabler relay 26 is preferably quite slow releasing so asto improve the'regulation on speech.

The initial sensitivity of the gain-increase disabler by suitable choiceof the values of its circuit elements is made approximately the same asthe sensitivity of the gain decreaser. lVhenever the average amplitudelevel of the amplified signals in the output of the amplifier 1 exceedsthe predetermined value, the currents diverted into control circuit 16cause the operation of the gain-increase disabler to open the contacts30 of relay 26 to disconnect the discharge resistcausing the gain of'the.vario-repeater to be increased. Thus the gain-increase disableroperates to nullify the action of the gain .in-

creaser for amplitude levels above the predetermined value. 7

Connected across the incoming signal transmission circuit 2 in the inputof vario-repeater. in the amplifier 1 is the input of another controlcircuit 31, which will be referred to hereafter as the gain increaser,comprising the three-electrode vacuum tube amplifying device 32 and thethree-electrode space discharge detector tube 33 connected in tandem,and the operating winding 34 of the master gain increase relay M and theoperating winding 35 of the auxiliary relay A connected in series acrossthe output of the rectifying device 33. The sensitivity of the gainincreaser by suitable choice of its circuit elements is set so thatwhenever the amplitude level of the signal waves in the output ofamplifier 1 exceeds a predetermined value corresponding to the averagepeaks of theweakest talkers, the portion of said currents diverted intocontrol circuit 31 causes the gain increaser to operate to energize theoperating winding 34 of the gain increase relay M. The energization ofthe operating winding of relay M causes the relay to operate to closethe normally open contacts 34. The contacts 34 when closed connect thecondenser 17 to ground through the discharge resistance R and the closedcontacts 30. of

disabler relay 26 when the latter relay is unoperated. The condenser 17then will discharge through the resistance R causing the in of thevario-repeater to be increased. The detector tube 33 is preferably agasfilled trigger tube. The gain increaser relay M is, hence,fast-operating, peak-actu at ed. It is preferably made comparativelyslow releasing. w

The circuits which have just been described,

i. e., the gain decreaser, the gain increaser and the gain-increasedisabler in combina-I tion with the vario-repeater form a volume controlcircuit which is not the invention of applicant. 7 v

The chain of auxiliary relays A A and A and associated circuitsdescribed below in combination with the master gain increase relay M areassociated with the volume control circuit, in accordance with thepresent invention, in such manner as to prevent false operation of thevolume control circuit by noise from increasing the gain of thevario-repeater when signals are not being received in the incomingsignal transmission circuit 2, and without altering the effectiveness ofthe circuit to decrease the gain when incoming signals over-ridethenoise, in the following manner The master relay M has a biasing winding36 which is maintained energized at all times by current from thebattery 37 through the resistance 40 so that the relay M would operatein the least sensitive condition were it not for a third winding 39thereon. The

third winding 39. of relay M is normally supplied with energi-zing,current from the battery 37'through the resistance 38 and the nor-'mally closed contacts 41 ofanother auxiliary relay A thethird winding39 of relay M be ing so wound as to oppose thetactionof the operatingwindingv 34 thereof- The third winding 39, is utilized to apply backbias .to the relay M. opposing the normal bias produced by'the'biasingwinding 36, the amount of back bias normally applied being sufficient to.allow therelay M to operhte in itsmost sensitive condition. Thisnormally applied back bias is removed in one step when a biasing winding42and an operating wind ing 43. The biasing winding 42 isat all timesenergized by current from battery44 through the resistance-45.The-operating winding 43 of relay-A is normally ener .gized by currentfrom thebattery 44 overa path extending from ground T through battery44, resistance 46, winding 43 of the relay A and the normallyclosedcontacts 47 of a third auxiliary relay A to -ground,'so as to holdthe contacts 41 of relay A in the closed condition. I p Y A condenser 48and a resistance 49 in series are connected across the circuitcomprising the winding 43 0f relay A and the resistance 46 in series, sothat the condenser 48 will be charges through that winding and theresistances 46 and 49. This holdover period of time may be made of anydesired length by a proper choice of the values of condenser 48 and theresistances 46 and 49 to regulate the time constants of the relay Aconstants of relayA are preferablymade such that the relay will notrelease unless the contacts 47 of auxiliary relay A remain open for atime longer than the average speech syllable. Thus,the contacts 47 willnever be closed long enough by operation of relay A when speech energy,alone is supplied to the precedingrelay A in the chain to cause relay Ato release. The speech waves applied to relay A then will not beefiecti'veto cause The time I normally chargedby current from battery 1i any reduction in the sensitivity of the master relay M through relaysA and A Auxiliary relay A has two -windings,'a

biasing winding 50 and an operating winding the battery 52 when thenormally open contacts 54 of auxiliary relay A are closed byoperationofthe latter relay over a path extending from ground throughbattery 52, resistance 57 winding 51, closed contacts 54 to. ground.

A resistance 55 and a condenser 56 are connected in series across thecircuit comprising the winding 51 of relay A and the series resistance57. When the contacts 54 of relay A are closed, the current from thebattery 52 besides energizing the operating winding 51 of relay A alsocharges the condenser 56 through resistance 55. When the energizingcircuit for the winding 51 of relay A is broken by the openingofcontacts 54 of relay A the condenser 56 slowly discharges throughresistances 55 and 57 and the winding 51, causing the winding 51 to bemaintained energized, and thus therelay A to be maintained operated tohold the contacts 47 open, for a definite holdover period after thesupply of energizing'current to winding 51 from battery 52 ceases. Thevalue of the condenser 56 and the values of the resistances and 57 arechosen so as to give the relay A a holdover in its operation which issuflicient tobridge the intervals between peaks of intermittent noise ofa particular type which will be referred to below. y

The auxiliary relay A has, in addition to its operating winding 35, abiasing winding 58 which is always maintained energized by current fromthe battery 59 through the resistance 60. The values of'the battery 59and the resistance 60 are preferably chosen so that the relay A is aboutthree decibels more sensitive than themaster relay M. I r

A condenser 61 of suitable value is con nected across the operatingwinding 34 of master relay M and the winding 35 of-relay A to preventchattering of the contacts of these relays.

The operation of the system of Fig. 1 as a whole will now be described.

It will be assumed that speech waves are being received over theincoming circuit 2 which is subject to a varying amount of line noise. a

Thereceived speech and noise waves will be divided between theamplifying circuit 1 and the input of the control circuit 31. The mainportion of these waves will be amplifired by the multistage amplifier 1and the amplified waves transmitted to the outgoing circuit-'3. I p

The portion of the incoming speech and noise waves diverted intothe'control circuit 31 will be amplified by the amplifying device '32therein and rectified by therectifying device 33 connected tothe outputof the amplifying device 32. The resultingrectified cur rents in theoutput ofthe rectifying device 33 will be transmitted through theoperating winding 34 of the master relay M and the operating winding 35of the auxiliary relay A The master relay M and the auxiliary relay Aare designed to operate at a syllabic. frequency. The frequency of theiroperation depends upon the sensitivity of the associatedamplifier-rectifier devices, the type of the i111.- pressed waves andtheir rapidity and various other factors including the relay design."Auxiliary relay A being more sensitive than. relay M, will operate inadvance of relay M to start the chain of operations which willbedescribed later.

If the amplitude level of the-incoming signals diverted into the controlcircuit 31 is above a predetermined value corresponding to the averagepeaks of the weakest talkers, the rectified current passing through theoperating winding of the relay Mwill be sufficient to cause that relayto operate to' close its normally open contacts 34.. The closing of thecontacts 34 will connect the discharge resistance R to one terminal ofthe control condenser 17 in the common grid 'circuit'for the push-pulltubes 4 and 5 in the amplifier 1, and, if the normally closed contacts3O of theme chanical relay 26 have not been opened by the N resistance'Rand the contacts 30 of relay 26;

The charge on the condenser 17 will then leak off through theresistanceR to ground gradually reducing. the negative bias applied tothegrids .of'the push-pull tubes 4 and 5 in the vari0-repeater so as tocause the gai-n'of the vario-repeater to be raised.

Because of its greater sensitivity the auxil i'ary relay A will'oper'atein response to the rectified speech waves impressed upon its operatingwinding 35 slightly before the master iir relay M operates. If the noiseimpressed on I the relay A is continuous and above a given amount, thecontacts 54 will close and remain closed. l/Vhen contacts 54 are closed,the operating winding 51 of auxiliary relay A is energized by currentfrom battery 52 over a path extending froinground through battery 52,resistance 57, winding 51 and the closed contacts 54 of relay A toground. Relay Ar will then operate to open its contacts 47 to break thenormal energizing'circuit for the operating winding 43 of auxiliaryrelay A By proper selection of the values of the condenser 56 andresistances 55 and 57 comprising the holdover circuit for relay A andthe values of the condenser 48 and resistances 49 and 46 comprising theholdover circuit for relay A the holdovers-of these two relays are madesuch that relay A will not release for operation of relay A by thesyllabic variations in speech but will release for operation of relay Aon intermittent noise of a certain kind.

The A relay is provided to introduce a holdover action to take care ofcertain types of noise especially those With pronounced low frequencycomponents which would cause auxiliary relay A to vibrate freely withouteffecting operation of the auxiliary relay A The holdover time ofauxiliary relay A preferably should be greater than 0.01 seconds tobridge the interval between cycle noise peaks.

The noise waves impressed with the speech waves from the output of therectifier 33 in the gain increase circuit upon the operating winding ofauxiliary relay A are steady compared to the fluctuations in the speechwaves. The noise waves will cause the contacts 54 of relay A to bemaintained closed long enough to cause such operation of auxiliary relayA that its contacts 47 will remain open fora suflicient length of timeto enable relay A to release to open its contacts 41. The opening of thecontacts 41 will break the energizing circuit for winding 39 of masterrelay M thus removing the normal back bias from the latter relay. Theremoval of the back bias from the relay M in response to the noise wavesin the manner pointed out will cause relay M to be made suflicientlyless sensitive so as just not to be falsely operated by noise aloneimpressed upon its operating winding, While still being responsive tothe impressed speech waves when the latter over-ride the noise.

The means provided for varying the sensitivity of the master relay M soas to prevent its false operation by line noise when no speech is beingtransmitted over the main transmission path does not alter theeffectiveness of the means for decreasing the gain of the vario-repeaterwhen the input level thereto increases above the predetermined value,for the operation of the gain decreaser is entirely independent of theoperation of the gain increaser as will be shown.

Now let it be supposed that the level of the amplified signals in theoutput of the amplifier 1 in the main signal transmission path increasesabove the predetermined value at which the output level is to bemaintained substantially constant. The sensitivity of the gaindecreaserbranch 18 and that of the gain-increase disabler branch 18 areadjusted so that the amplifier detectors in these branches will beoperated for input waves above that predetermined level diverted intocontrol circuit 16. The rectified speech waves in the output of the fullwave rectifier 20in the gain decreaser circuit willbe impressed acrossthe control condenser 17 m the varlo-repeater input clrcuit causlngcircuit.

repeater until the levelof the amplified waves in the output of theamplifier 1 falls below the operating value of the gain decreaser Hence,the average level of the speech waves transmitted to the outgoing signaltransmisison circuit 3 will be main- 1 tained substantially constant forwide variations in the volume levels of signals impressed on the inputof theamplifier circuit 1. The gain decreaser in the circuitwhich branch19 are transmitted through the operating winding 25 of the relay '26causing the has been described operates by reducing the =24. Therectified speech waves inthe'output of the detectortube 24 in thecontrol,;

operation of that relay. The operation of V relay 26 causes its normallyclosed contacts 30 to open to disconnect the normal ground from thedischarge resistance R. Subsequent operation of-the gain increase relayM will,

therefore, not result in any increasein the gain of the vario-repeatersinceground has been efi'ectivelyremoved from the discharge resistance Rand, thenefore, fromthe control condenser circuit. I

In the manner which has just been described, the relay circuits shown inFig. 1' operate when there is noise'present in the incoming signaltransmission circuit above a certain level to reduce in one step thesensitivity of the master gain increase relay M to the point where itwill just not be operatedby the impressed noise. It is apparent, ofcourse, that a closer approximation of continuous 3 variation of thesensitivity of the master relay with noise may be obtained by usingadditional relays to cause the sensitivity .to be changed in smallsteps. This may be obtained, for example, asshown in Fig. 2 by providingadditional steps in back bias on the master relay Mcontrolled by othersets of auxiliary relays, such as A A and A corresponding to auxiliaryrelays A A and A in the circuit of Fig. 1, each set of relays having therequired design to produce a different proportion of the reduction inthe sensitivity of the master relay.

In Fig. 2. the relays A i to A are identical Withrelavs A1 to A exceptthat byvsuitable choice of the biasingelements thereof the relay A,is'made less sensitive than the relay series with the biasing winding 50of relay A is energized by current from battery 52 through resistance53. The operating winding 51 of relay A is arranged tobe energized bycurrent from battery 52 through resistance 57 whenthe normally opencontacts 54 of relay A are closed by operation of'relay A I The biasingwinding 42' of auxiliary relay Ag inseries with the biasing winding 42of relay A is energized by current from battery 44 through theresistance 45. The operating 21; winding 43 of relay A is normallyenergized by current from battery 44 over a path extending from groundthrough battery 44,

resistance 45, winding 43 of relay A and the normally closed contacts 47of auxiliary relay A to ground, so as to hold the contacts 41 of relay Aclosed; Similarly, the operating winding 43 of relay A is normallyenergized by current from battery 44 over a path extending from groundthrough battery 44, resistance 46, winding 43 of relay A and thenormally closed contacts 47 of relay A to ground, so as to hold relay"contacts 41 of relay A closed.

As in Fig. 1, the biasing winding 36 of master relay M is energized at'all times by current from the battery 37 through resistance 40 so thatthe relay would operate in the least sensitive condition were it not forthe third winding 39. The third winding 39 of relay M so wound as tooppose the action of the operating winding 34 thereof, is normallyenergized by current from battery 37 over a path extending from groundthrough battery 37, resistance 38, winding 39, normallyclosed contacts41 of relay A and normally closed contacts 41 of relay A to ground, thussup.

plying back bias to the relay M opposing the normal bias produced by thebiasing winding 36 of that relay. The value of the elements in theseries circuit are. so chosen that the amount of back bias normallyapplied is sufiicient to allow the relay M to operate in its mostsensitive condition.

The sensitivity of the auxiliary relay A is adjusted by means of theresistance shunting its winding 58 so that the relay A will operate toclose its contacts 54 whenever the amount of continuous noiseimpressedupon its operating winding 35 from the output of the rectifying device33 exceeds a given low value. Also, the sensitivity of auxiliary i'elayA is adjusted by suitable choice of biasing elements so that the relaywill operate to close its contacts 54 when the amount of continuousnoise impressed upon its operating winding 35 from the output of therectifyin device 33 exceeds a given higher value.

Tiet it be assumed that noise is present in the incoming signaltransmission path 2 in sufficient amount so that when-it is amplified bytheamplifying device 32 and rectifiedby the rectifying device 33 incontrol circuit31, it will cause the operation of relay A but not theoperation of relay A This will start the chain of operations,asdescribed above in connection with F ig. 1, culminating in'the releaseof the relay A to open its normally closed contacts 41.

The opening of contacts 41 will remove the normal short circuit aroundthe resistance thus connecting that resistancein the energizing circuitfor the third winding 39 of master relay M, the energizing circuitforvthat relay winding now extending from ground through battery 37,resistance 38, winding 39 of relay M, the normally closed contacts 41'of relay A and resistance 7 0 to ground. The value of the resistance 70is so selected that the amount of back bias removed from the relay M byits inclusion in the energizing cir- "cui't for its third winding, isjust sufiicient to prevent operation of that relay by noise of saidgiven low amount impressed on its operating winding, while allowing itsoperation when the signals over-ride the noise.

Now let it be supposed that the amount of noise impressed from theoutput of the rectifying device 33 on the series circuit comprising theoperating winding 35' of relay A the operating winding 35 of relay A andthe operating winding 34 of relay M increases to such a value as tocause the operationof relay A in addition to relay A causing thecontacts 54 of relay A to close. This will start the chain of operationsculminating inv the opening of normally closed contacts 41 of auxiliaryrelay A through the operation of relay A and the consequent releaseofrelayA The opening of contacts ,41 of relay A will remove the normalshort circuit around resistance 70, causing that resistance to beconnected in series with the resistance 70 in the energizing circuit forthe third winding higher value impressed .upon its operating winding. Bythe use of additional chains of relays corresponding to the relays A toA and additional resistances corresponding'to resistances 70 and 70' andconnected in similar manner, it may be seen that the sensi-o tivity ofthe master relay M may be changed instill smaller steps, so as to obtaina closer approximation of continuous variation of sensitivity with theamount of noise.

The auxiliary relays A A may be dispensed with in systems not subject tonoises of the type for which those relays are provided, that is, noiseswhich have pronounced low frequency components. If the relays A Aare'not used, the winding of the auxiliary relay A would be controlleddirectly from the contacts of the auxiliary relay A and A the winding ofauxiliary relay A would be controlled directly from the contacts ofauxiliary relay A The noise desensitization circuits of the in ventionhave been described as applied to a particular type of volume controlcircuit in which the apparatus is arranged in a particular way. It isapparent, however, that the invention would be applicable to types ofvolume control circuits other than that shown, and that variousmodifications 1n the noise desensitization circuit may be made bypersons skilled in the art without departing from the spirit and scopeof the invention as defined in the appended claims.

lVhat is claimed, is: j

1. In a signal transmission system, a signal trasmission path subject tointerfering noise' waves, wavecontrolled means connected to said pathand responsive to waves there n above a given level to ad ust the gainof sald path, and means under control of the noise waves in said pathabove said certain level, for reducing the operating sensitivity of thegain adjusting means to prevent its false operation by said noise wavesalone.

2. In a signal transmission system, a signal transmission path subjectto interfering noise waves, wave-controlled means connected to said pathand responsive to the waves transmitted thereover when they exceed acertain minimum level, to increase the gain of said path, and means forautomatically reducing the operating sensitivity of said wavecontrolledmeans to operation when the amount of noise in said path increases abovea given value, to prevent false operation by noise.

3. In a signal transmission system, a signal transmission path subjectto interfering noise waves, wave-controlled means connected to said pathand responsive to an increase 1n the volume level of the signals thereinabove a given level to adjust the gain of said path, and means undercont-r01 of the noise waves in said path above a certain level forreducing the operating sensitivity of the gain adjusting means inproportion to the amount of noise in said path to prevent falseoperation of said gain adjusting means by noise 'WEIVGS, wave-controlledmeans connected to go said path and responsive to the signal wavestransmitted over said path'when they exceed a predetermined level toincrease the gain of said path, and means for reducing the ope-ratingsensitivity of said wave-controlled 15 means in proportion to the amountof noise in sa d path when it increases above a given value, to preventfalse operation by said noise.

5. In a signal transmission system in coma bination with a signaltransmission path subi0 ject to interfering noise waves, wave-controlledmeans connected to said path and responslve to waves therein above agiven low volume level, toincrease the gain of said n path, meansconnected to said path and re- 15 sponsive to signals therein when theyexceed a certain higher volume level to reduce the gain of said path,and means connected to said path and responsive to the signals therer Iu u I v v v s 1n exceeding said higher volume level, to disable saidgain increasing means, and means responsive to noise in said path abovesaid low volume level, for reducing the sensitivity of the gainincreasing means to the point where it is just inoperative by id noisealone althou h operative by signals overriding the noise.

6. In a signal transmission system, a signal transmission path subjectto interfering b noise, wave-controlled relay means connected -ioo tosaid path and responsive to waves transmitted thereover when they exceeda certain low level to increase the gain of said path, means connectedto said path and responsive to the signals therein when their volumelevel 1 exceeds a given value higher than said certain low level, toprevent operation of said wave-controlled means from mcreasing the Igain of said path, and means responsive to the noise above said certainlow level in said path for continuously biasing said relay means to thepoint where it is'just inoperative by said noise alone althoughoperative by signals over-riding the noise.

In combination with a system for main-' 5 taining the volume of signalstransmitted over a signal transmission path subject to noisesubstantially constant at a given average level comprising an amplifierin said path for amplifying said signals, means connected to said pathin the input of the amplifier and responsive to the incoming wavesexceeding a predetermined level lower than said given level, to increasethe gain of said amplifier, means connected to said path in the outputof said amplifier and responsive to the amplified signals exceeding saidgiven level, to decrease the gain of said amplifier until the volume ofthe amplified signals is reduced to said given level, and means alsoconnected to said path in the output of said amplifier and responsive tothe amplified Waves when they exceed said given level, to disable saidgain increasing means, means responsive to llOlSG in said path abovesa1d predetermined level, for reducing the sensitivity of said gainincreasing means to maintain said gain increasing means inoperative bysaid noise alone although operative by said incoming signals over-ridingsaid noise.

8. The system of claim 7 and in whichsaid sensitivity reducing meanscomprises a chain of relays operating in sequence in response to thenoise Waves received from said path and having difierent operating andholdover times determined by the types of noises against whichprotection is desired, to bias said gain increasing means againstoperation by said noise according to the amplitude level of said noise.

9. The system of claim 7 and in WhlCh'Sitld gain increasing meanscomprises a relay controlled by the Waves in the input of said amplifierfor applying a biasing potential to said amplifier proportional to theinput level of the Waves impressed thereon, and said sensitivityreducing means comprising means unresponsive to said signals butresponsive to noise, for biasing said relay against operation bysaidnoise, and Without altering the effectiveness of said gain decreasingmeans.

10. The system of claim 7 and in which said amplifier comprises a spacedischarge amplifying device having a condenser connected between itsinput electrodes, the charge on said condenser controlling the gain ofsaid device,said gain decreasing means comprises means for rectifyingsaid incoming signals and applying the' rectified signals to charge saidcondenser in proportion to the signal amplitude level, said gainincreasing means comprises a relay which when operated causes saidcondenser to be discharged, and the sensitivity reducing means comprisesmeans controlled by the noise in said path and unresponsive to signalstherein, for biasing said relay against operation in proportion to theamount of noise on said path.

In Witness whereof, I hereunto subscribe my name, this 24 day of August1931;

ARTHUR W. HORTON, JR.

